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| Odd output from Elenco XP-15K Power Supply |
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| ArthurDent:
Great that you got your power supply kit working! Think of this minor problem as a positive learning experience. Future kits you assemble will seem easier now that you've had success with your first one. One change that has been made from when you (and I) soldered all those years ago is the use of lead free solder. If you read the construction section of your power supply manual on soldering they mention how the new lead free solder requires higher temperatures and doesn't flow as well. This obviously made your soldering job more difficult. There is a possibility that your small 25W iron doesn't get quite hot enough for lead free solder and that's why the higher temperature of your soldering gun melted the solder better even though it is physically way too big. Even if the iron gets to the required temperature you probably have to leave the tip on the spot you're soldering a few seconds longer to allow the solder to flow properly. Here is a link to a short video that compares soldering with lead free vs lead solder and has good information. Just remember that there are videos on everything you can think of and some can be very helpful so Google is your friend. Members on this forum are always ready and willing to help as well. One thing that might help is to get some old scrap circuit board to practice soldering on. sometimes you will find a thin layer of oxide or crud on a circuit board and lightly cleaning the board with a Scotch Brite pad will help the solder adhere. |
| tomherrick:
I think somebody else needs to own that 25W iron... I'll find another higher wattage unit that is more nimble around a circuit board than the heavy-watt gun. Oh, and I'll watch that video at lunch today. Thanks. I do want to limit the output to 15VDC, however, as the reason for acquiring the power supply is to test and set up marine electronics off the boat in the shop. 15VDC is the top end of the range for the Garmin chartplotter we acquired, and I'd expect that's about it for most nominal 12V equipment. I tested all the resistors: R1-2.1k, R2-147, R4-2.2. R3, the 2K pot, when turned all the way to the right at the 15V setting reads 2.38k. When turned to the left at the 0V setting it reads 0.00 ohms. I'm not getting something here. Why would a value of 0 ohms totally block the voltage output and 2.38k ohms allow almost 20VDC? rf+tech mentioned: "Limiting the output to 15 Volts would require adding a 10 kOhm resistor in parallel with the pot, to reduce the effective resistance to 1.65 kOhms." Looking at the schematic I'm not sure where such a resistor would go to be parallel with the pot. |
| rf+tech:
The extra resistor would be connected across the two ends of the potentiometer. As supplied, a 2 kOhm pot has too much resistance to limit the output to 15 Volts. The total value of the paralleled resistor/potentiometer is derived from the LM317 datasheet formula: Vout = 1.25V x (1 + (Rpot/R1)) + Iadj x Rpot The last term compensates for the current flow from the adjust terminal, which is small enough to ignore. So the simplified formula reduces to: Vout = 1.25V x (1 + (Rpot/R1)) Substituting 147 Ohms for R1 and solving for Rpot, we arrive at 1610 Ohms, which limits Vout to 15 Volts. By adding a resistance in parallel with the potentiometer, we can "trim" the maximum value. Take the reciprocal of 1610 Ohms, subtract the reciprocal of 2380 Ohms, then find the reciprocal of the result. That value represents the amount of parallel resistance to reduce the total to the calculated value of 1610 Ohms. As for the result of that calculation, I'll leave that as an exercise for the student. ;) Do post your result. RF+ Tech Edited for better clarity |
| tomherrick:
Which would be the two ends of the pot? There are three contacts, two appear to be from P1 and P3 by various routes, and one is an output leading to P5. |
| rstofer:
The center contact is usually the wiper. Connect your Ohmmeter across the two outer pins and note that the value doesn't change when you turn the shaft. Disconnected from the PCB, of course. https://components101.com/potentiometer |
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